1. Field of the Invention
The present invention relates to an ink cartridge that can store ink and an ink jet printing apparatus for performing printing on a print medium by using ink supplied from the ink cartridge.
2. Description of the Related Art
Ink using pigments as a colorant is commercially used in ink jet print heads. The pigments, however, are based on dispersion, which are different from dyes which are based on dissolution, and therefore it is known that pigment particles precipitate in an ink cartridge in which ink is stored.
More specifically, if an ink cartridge is left for a long period of time while being mounted on an ink jet printing apparatus, precipitation of pigment particles occurs in ink stored in the ink cartridge. This causes a concentration gradient of pigment particles in the ink stored in the ink cartridge from its bottom toward its top. As a result, at the bottom of the ink cartridge, a concentration of pigment particles is high and an excessively highly colored layer is formed, whereas at the top of the ink cartridge, a concentration of pigment particles is low and an excessively lightly colored layer is formed.
A description will be given of the case where the ink cartridge is stored in a fixed position (in a state in which its bottom faces down in a vertical direction) for a long period of time in the configuration in which ink is delivered from the bottom of the ink cartridge. If the ink delivered from the bottom of the ink cartridge is supplied to a print head, ink forming a layer of a high concentration of pigment particles is supplied first, resulting in printing an excessively highly colored image. In other words, density variations which are visually recognized may be generated between a print image formed at an early stage of use of the ink cartridge and a print image formed at a later stage of use of the ink cartridge. Such a phenomenon is particularly conspicuous in color printing in which a color image is printed utilizing gradations of color.
A method for solving the above problem is disclosed in Japanese Patent Laid-Open No. 2005-007855, in which a connection portion for delivering ink in an ink cartridge is provided at the bottom of the ink cartridge, and a tube extending upwardly in a vertical direction is provided in the connection portion. The tube is provided with a plurality of ink inflow ports which communicate with the ink cartridge in their respective positions in the vertical direction, and the ink inflow port located at a lower part in the vertical direction has a greater inflow resistance as compared to the other ink inflow ports.
This allows the ink to flow into the tube from a position in which a concentration of pigment particles is high and a position in which a concentration of pigment particles is low in the ink cartridge in amounts corresponding to their respective inflow resistances, and the ink mixed in the tube is delivered from a delivery port formed at a lower end of the tube.
Furthermore, Japanese Patent Laid-Open No. 2006-102971 discloses a liquid injection apparatus having another technique to solve density variations generated between an image formed at an early stage of use of the cartridge and an image formed at a later stage of use of the cartridge. The liquid injection apparatus disclosed in Japanese Patent Laid-Open No. 2006-102971 is intended for uniformity of ink concentration in the cartridge by drawing liquid in a liquid reservoir space into a circulation channel and returning the liquid from the circulation channel to the liquid reservoir space to stir the liquid.